Question

In adsorption, the Langmuir isotherm assumes:

• A. Multi-layer adsorption
• B. Monolayer adsorption with uniform surface energy
• C. BET theory applies
• D. Freundlich equation is valid

Hint:

To quickly distinguish between isotherms:
Langmuir = Monolayer + Homogeneous surface
Freundlich = Multilayer/Monolayer + Heterogeneous surface
BET = Multilayer + Homogeneous surface

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary physical assumptions behind the Langmuir adsorption isotherm model.
This is a core topic in mass transfer and surface chemistry.

Step 2: Key Formula or Approach:
The Langmuir adsorption isotherm model is given by:
\[ \theta = \frac{K \cdot P}{1 + K \cdot P} \]
where \( \theta \) is the fractional surface coverage, \( P \) is the partial pressure of the adsorbate, and \( K \) is the adsorption equilibrium constant.

Step 3: Detailed Explanation:

• Langmuir Model Assumptions:
1.

Monolayer Coverage: The solid adsorbent surface has a fixed number of active sites, and each site can hold only one adsorbate molecule.
Once a site is occupied, no further adsorption can occur at that location.
This prevents the formation of multiple layers.
2.

Uniform Surface Energy (Homogeneous Surface): All adsorption sites are energetically identical and equivalent.
The heat of adsorption is constant and independent of the surface coverage (\( \theta \)).
3.

No Intermolecular Interactions: Adsorbed molecules do not interact with neighboring molecules on adjacent sites.
4.

Reversibility: The process is a dynamic equilibrium between adsorption and desorption.

• Other Models:
-

BET Theory: Extends the Langmuir model to account for multi-layer adsorption.
-

Freundlich Isotherm: An empirical model that assumes a heterogeneous surface with varying adsorption energies.


Step 4: Final Answer:
The Langmuir isotherm assumes monolayer adsorption on a homogeneous surface with uniform surface energy.